1. Field of the Invention
The present invention relates to a transfer case used on a vehicle equipped with four wheel drive and, more particularly, to an apparatus for causing a transfer case to shift the vehicle between two wheel drive and four wheel drive conditions.
2. Description of Related Art
In recent years, various types of automotive vehicles with four wheel drive transmission systems (which are hereinafter referred to simply as four wheel drive vehicles) have become increasingly popular. Some types of these four wheel drive vehicles are adapted to be shifted between two wheel and four wheel drive with a differential locked or unlocked condition. To manually shift such a four wheel drive vehicle among two wheel drive, four wheel drive with a differential locked condition and four wheel drive with a differential unlocked condition, a transmission is provided with a center differential having a shift mechanism provided with a shift member which is either manually operated or motor driven to move among three axial positions. Such a center differential is known from Japanese Utility Model publication No. 60(1985)-127,232 entitled "Apparatus For Dividing Power For Automotive Vehicle", laid open on Aug. 27, 1985.
One serious problem associated with the center differential having a motor driven shift mechanism is an interruption of smooth movement of the shift member to desired axial positions for some reasons which increases a load on a motor. To prevent a reversible motor of the shift mechanism from being subjected to such an excessive load, it was thought to provide a load detecting means which is actuated when loads larger than a predetermined load are exerted on the motor to shut off the supply of power to the motor and thereby stop the motor. The load detecting means is adapted to detect the predetermined load on the motor rotating in a normal direction or a reverse direction. However, if the load detecting means can not sense the direction of rotation of the motor, it sometimes happens that the load detecting means makes a misjudgement and allows the motor to stop when there is no excessive load on the motor. That is, there are many cases of shifting back the shift member to one axial position immediately after having shifted it from one to another axial position. In such cases, although the motor should be quickly reversed at the moment that the shift member is fully shifted to the other axial position, the motor can possibly over run in one direction in which it shifts the shift member toward the other axial position due to inertia of the motor, so as to actuate the load detecting means. If in fact the load detecting means is actuated due to the over running of the motor, the motor, although being provided with a reverse signal, is disabled from rotating in the other direction, and thereby it is impossible to shift the shift member to the other axial position so as to shift the drive condition of the vehicle. Although the problem may be of course successfully eliminated by providing the load detecting means for each direction of rotation of the motor, the provision of two load detecting means is unfavorable in structure and cost.